1. Field of the Invention
The present invention relates to a device for high performance coding a plurality of moving image signals and broadcasting or transmitting them simultaneously and, particularly, to a moving image coding device thereof, which uses an inter-frame or inter-field prediction.
2. Description of the Prior Art
Besides an analog broadcasting system such as NTSC, a technique in which a moving image signal is digitized and a resultant digital data stream is broadcast through a satellite broadcasting, ground wave broadcasting or CATV system is having been studied.
In such technique, when a high performance coding method is used to digitize the moving image signal, it is possible to transmit a plurality of images through a transmission path of a conventional one image channel. For example, a transmission path for transmitting an FM modulated NTSC signal has a band width of about 27 MHz. This band width allows a digital data of about 30 Mbps to be transmitted. Since, when the NTSC signal is coded by using the motion compensation inter-frame prediction, a sufficient image quality is obtainable by about 10 Mbps, it is possible to transmit three kinds of moving image through the transmission path of 30 Mbps.
When such broadcasting signal is recorded by a digital VTR, it can be considered to record a code train as it is and to decode it when it is to be reproduced. In such case, if the signal is coded by using the inter-frame prediction, it is impossible to perform a high speed search for reproducing only a specified frame in the successive frames. In order to solve this problem, one frame per several frames is independently intra-frame coded according to, for example, MPEG (Moving Picture Experts Group) system standardized by ISO (International Organization for Standardization). In this case, although the inter-frame prediction coding is usually a cyclic prediction, an accumulation of error due to signal processing error and/or transmission code error is prevented by interrupting the inter-frame prediction periodically and intra-frame coding independently. This scheme is referred to as refresh of error. A frame which is intra-frame coded independently is referred to as I frame. In the high speed search, only the I frame is reproduced. Although the term "frame" can be changed to "field", the following description will be made with reference to "frame".
Conventional Coding Device
FIG. 1 is a block diagram of an example of a conventional moving picture coding device for digitally broadcasting two kinds (2 channel) of picture signal. There are provided two coders which are basically the same and code the respective moving pictures.
In FIG. 1, a signal of a picture a inputted from an input terminal 1 is supplied to a subtracter 5 and a picture type setting device 41. A signal of a picture b inputted from an input terminal 11 is supplied to a subtracter 15 and a picture type setting device 42.
The subtracter 5 subtracts a prediction signal from the input signal a. A residual signal obtained by the subtracter 5 is supplied to a coder 6. In the coder 6, the DCT (Discrete Cosine Transform) is performed. A coefficient obtained by the DCT is quantized with a predetermined step width and variable-length coded. A data train thus compressed is supplied to a buffer 43 and a decoder 8. The buffer 43 absorbs a variation of the amount of code caused by the variable-length coding to make a transfer rate of the data train substantially constant and supplies it to a multiplexer 100. The decoder 8 performs a processing which is reverse to that performed by the coder 6 and outputs a reproduced residual signal. An adder 7 adds a prediction signal supplied from an inter-frame predictor 3 to the reproduced residual signal supplied from the decoder 8. The reproduced picture signal output from the adder 7 is sent to the inter-frame predictor 3.
The inter-frame predictor 3 delays the reproduced picture signal by 1 frame and outputs it as a inter-frame prediction signal which is supplied to the subtracter 5 and the adder 7 through a switch 4.
A movable terminal of the switch 4 is connected to the inter-frame predictor 3 when a picture type information supplied from the picture type setting device 41 indicates a unidirectionally predictive frame (P frame: Predictive-Frame) and is connected to the 0 value and cuts the predictive signal when the picture type is an intra-frame (I frame). When the predictive signal is cut, the input picture signal is supplied to the coder 6 as it is, so that the intra-frame independent coding is performed.
The P frame means a frame which is coded by the inter-frame prediction coding by using a predictive signal produced from a preceding frame and the I frame means a frame which is intra-frame coded independently without using the inter-frame prediction.
The selection of a frame to be set as the I frame is performed by the picture type setting device 41 with a frame sync timing of the input picture. The picture type information from the picture type setting device 41 is supplied to the switch 4 and multiplexed with the data train from the buffer 43 by the multiplexer 100 and sent to a decoding device.
The timing of the I frame setting may be every 20 to 30 frames for a high speed search and/or error refresh during a VTR recording. In this case, since the amount of code of the I frame is 5 to 10 times that of the P frame, the amount of code generated varies frame to frame considerably. In order to absorb such large variation, a buffer having large capacity is required.
Alternatively, there is a method in which an independent picture is set not frame unit but a portion of a frame which may be one tenth of one frame and the portion is shifted every refresh so that the whole image is refreshed for about 1 second. In such case, the buffer capacity may be small. However, a search picture during a VTR recording is divided, causing a monitor to be difficult.
Operations of a subtracter 15, a coder 16, a buffer 44, a decoder 18, an adder 17, an inter-frame predictor 13, a switch 14 and the picture type setting device 42 for the picture b are the same as those of the subtracter 5, the coder 6, the buffer 43, the decoder 8, the adder 7, the inter-frame predictor 3, the switch 4 and the picture type setting device 41 for the picture a, respectively.
The multiplexer 100 multiplexes the output of the buffer 43, which is the data train of the picture a, with an output of the buffer 44, which is a data train of the picture b, and outputs a multiplexed data train from a data output terminal 20.
Conventional Decoding Device
An example of a conventional decoding device for reproducing a data train supplied thereto will be described with reference to FIG. 2.
The decoding device shown in FIG. 2 is one to be associated with the coding device shown in FIG. 1. In FIG. 2, a multiplexed data train input from a data input terminal 31 is demultiplexed to data trains of the pictures a and b. The data train of the picture a is supplied to a buffer 53 and the data train of the picture b is supplied to a buffer 64.
The data train of the picture a is written in the buffer 53 at a constant speed, read out therefrom at a speed matched with a timing of a decoding processing to be performed in a decoder 58 and supplied to the decoder 58. The decoder 58 performs a similar processing to that performed in the coding device shown in FIG. 1 and outputs a reproduced residual signal. The reproduced residual signal is supplied to an adder 57.
The adder 57 adds a predictive signal supplied from an inter-frame predictor 55 to the reproduced residual signal and outputs a reproduced picture signal of the picture a. The reproduced picture signal is supplied to the inter-frame predictor 55 and an output terminal 59.
The inter-frame predictor 55 delays the reproduced picture signal by 1 frame and outputs it as an inter-frame predictive signal. The inter-frame predictive signal is supplied to the adder 57 through a switch 54. The switch 54 cuts the inter-frame predictive signal according to the picture type information separated by a demultiplexer 33.
On the other hand, operations of a buffer 63, a decoder 68, an adder 67 and an inter-frame predictor 65 are the same as those of the buffer 53, the decoder 58, the adder 57 and the inter-frame predictor 55, respectively, and the reproduced picture signal of the picture b is output from an output terminal 69.
The buffer capacity corresponds to a maximum variation with respect to a fixed transfer rate of the generated code amount. The larger the variation requires the larger the capacity.
On the other hand, since the coding and decoding processing is performed in synchronism with the input of the moving picture signal, the delay time between the data input to the buffer on the coding side and the data output from the buffer on the decoding side becomes constant. That is, when the delay is large due to accumulation of a large amount of data in the coding side buffer, the amount of data accumulated in the decoding side buffer and hence the delay is small. On the contrary, when the delay is small due to small amount of data accumulated in the coding side buffer, the amount of data accumulated in the decoding side buffer and hence the delay becomes large. Therefore, a total amount of data accumulated in the both buffers is equal to the capacity of one buffer. Further, the capacity of the coding side buffer is the same as that of the decoding side buffer and the both buffers operate complementarily.
From the foregoing, it is clear that the amount of delay is a quotient of a division of the capacity of one buffer by the transfer rate. For example, when the buffer capacity is 1 Mbit and the transfer rate is 5 Mbps, the delay becomes 0.2 seconds. With the same buffer capacity, the lower the transfer rate provides the larger the delay and, with the same transfer rate, the larger the buffer capacity provides the larger the delay.
In the inter-frame prediction coding, the amount of code of the intra-frame (I frame) is 5 to 10 times that of the unidirectionally predictive frame (P frame). Therefore, the generated code amount varies substantially frame to frame and, in order to absorb such large variation, the buffers having large capacity are required, by which the signal delay becomes large.
When a portion of the frame is independently coded in order to reduce the code amount variation, a high speed search picture during a VTR recording/reproducing is divided, causing a monitor to be difficult.
Further, the code amount of the unidirectionally predictive frame (P frame) in which the inter-frame prediction is performed by using a predictive signal produced from a preceding frame in time is 2 to 5 times that of the bidirectionally predictive frame (B frame) in which the inter-frame prediction is performed by using a preceding and succeeding frames in time. Therefore, in order to absorb the variation, buffers having large capacity are required as in the case of the intra-frame (I frame).
Further, the same problem exists even if the frame unit is changed to the field unit.